Cómo obtener datos de un usuario de WordPress

memberperks.php

<?php
/*
Plugin Name: MemberPerks
Description: Ofrece beneficios a tus usuarios más leales
Author: David Perálvarez
Version: 1.0.0
Author URI: https://silicodevalley.com
*/

// Create the shortcode
add_shortcode( 'show_member_level', 'mp_show_member_level');

// Get the member level
function mp_show_member_level(){

  // Get current user's ID
  $memberID = get_current_user_id();

  if( $memberID != 0 ):

    // Get member's info
    $memberInfo = get_userdata($memberID);
    $memberName = $memberInfo->first_name;
    if(empty($memberName)):
      $memberName = $memberInfo->user_login;
    endif;

    // Registration date
    $memberRegistered = $memberInfo->user_registered;
    $registrationDate = new DateTime($memberRegistered);

    // Current Date
    $currentDate = new DateTime("now", new DateTimeZone('Europe/Madrid'));

    //https://stackoverflow.com/questions/4233605/elegant-way-to-get-the-count-of-months-between-two-dates
    $memberLevel = $registrationDate->diff($currentDate)->m + ($registrationDate->diff($currentDate)->y*12) + 1;

    ob_start();
    ?>
    <p class="mp-member-level">
    <?php echo $memberName; ?>
    tu <strong>nivel de perturbación en la fuerza es de
    <?php echo $memberLevel; ?></strong>,
    pronto te explicaré de qué va esto :)
    </p>
    <?php
    $output = ob_get_contents();
    ob_end_clean();
    return $output;

  else:
    return; // User is not logged in
  endif;

}

// Include styles
add_action( 'wp_enqueue_scripts', 'mp_add_styles' );

function mp_add_styles(){
  wp_register_style( 'mp-styles', plugin_dir_url(__FILE__) . 'assets/css/mp-styles.css' );
  wp_enqueue_style( 'mp-styles' );
}